106s use torsion bar rear suspension,, so the mounts bolt thru the boot floor, closer to the middle of the car.... not in the areas that has rotted/torn

in this picture the Rear mounts for the beam are under the little black plastic bung in the floor. IIRC.




so that would appear to be just normal rot? If it was mega stiff suspension causing it. then surely the main points of Tearing would be around the mounting points to the beam... from them plastic covers and the one in front of it (almost under where the rear seats should sit)

EDIT: having said that, i can see how Maybe it could try and bring the whole boot floor upwards like that